1. Field of the Invention
The present invention relates to a vehicle navigation system, and more particularly relates to a method and an apparatus for estimating a time delay of a GPS receiver in a hybrid navigation system.
2. Description of the Related Art
Navigation systems, which determine positions of movable bodies, and search for paths to a destination so as to provide an optimal path, are typically mounted on movable bodies such as ships, airplanes, or vehicles. Further, in order to determine the position of the movable body and provide the optimal path to the destination, a navigation system must be able to determine an accurate position of the movable body.
Accordingly, navigation systems typically have a positioning apparatus for determining their own position. The positioning apparatus is classified into an apparatus of determining a position with exterior help and an apparatus of determining a position by means of an interior sensor. A representative example of the former is a global positioning system (hereinafter, referred to as a GPS) and a representative example of the latter is a dead reckoning system (hereinafter, referred to as a DR) using an inertial sensor.
A DR system (so-called “inertial navigation system”) including the inertial sensor is a navigation system which had been initially developed by the Massachusetts Institute of Technology in the USA in the early 1950's and had been put to practical use in the 1960's. The DR system obtains a velocity by means of a speedometer or an accelerometer, and obtains a movement direction by means of a gyroscope for detecting a rotation motion of a vehicle and a magnetic compass for measuring an azimuth angle by using the Earth's magnetic field. That is, the DR system integrates an output of the gyroscope by measuring a rotation angular velocity to obtain a travel direction angle of the vehicle, and compensates for an acceleration of gravity from an output of the accelerometer and integrates the compensated signal, thereby autonomously calculating the current velocity and position of the vehicle.
The DR system can provide accurate and continuous navigation data for a short period. However, the DR system has a disadvantage in that due to the integration process, an error is accumulated over the passage of time.
Meanwhile, the GPS is far behind the DR system in terms of momentary accuracy, but an error is not accumulated over the passage of time.
Accordingly, in order to secure higher accuracy and long-term stability, most DR systems are not used alone, but are used together with a non-inertial auxiliary sensor such as a magnetic compass or a GPS. This type of navigation system, using an inertial sensor and a non-inertial sensor, is called a hybrid navigation system. From among hybrid navigation systems, a GPS/DR system is designed to have advantages of the DR system and the GPS by combining the DR system and a GPS receiver into one system.
The GPS/DR system may have a loosely-coupled structure or a tightly-coupled structure according to the desired design. In the loosely-coupled structure, the GPS receiver and the DR system are maintained as respective independent systems and their respective outputs are combined. In contrast, in the tightly-coupled structure, a measured value, such as a pseudo-range provided from a GPS satellite to the GPS receiver, and measured values of an angular velocity and an acceleration, which are obtained from the inertial sensor, are internally combined. In order to maximize the coupling of the GPS and the DR system, the tightly-coupled structure is adequate. However, the loosely-coupled structure has a design that is simple and therefore realization is easy. In addition, since sufficient performance may be obtained with the loosely-coupled structure, the loosely-coupled structure is generally used in the navigation system.
FIG. 1 is a block diagram schematically showing a conventional hybrid navigation system. Referring to FIG. 1, the hybrid navigation system 10 includes a DR system 12, a GPS receiver 14, and a combination filter 16. Since the hybrid navigation system 10 includes the DR system 12 and the GPS receiver 14, it is also called a GPS/DR system. Each of the DR system 12 and the GPS receiver 14 calculates position/velocity/direction angle information, and sends this calculated information to the combination filter 16. The combination filter 16 combines the information to obtain a navigation solution of the hybrid navigation system. The DR system 12 uses an output fedback from the combination filter 16.
One of items which must be considered in the GPS/DR system is a time delay of information provided by the GPS. That is, the GPS receiver delays by a predetermined time information at a specific time to provide the delayed information, and therefore the GPS/DR system must consider the time delay when calculating the velocity of the vehicle.
FIG. 2 is a view showing a time delay of a GPS receiver in a conventional hybrid navigation system. Referring to FIG. 2, a velocity VG provided by the GPS receiver starts later than a velocity VA obtained from an accelerometer. That is, the velocity VG provided by the GPS receiver is delayed by a predetermined time.
Generally, the GPS receiver has a time delay of about 1 sec. However, when a tracking loop or a filter is used in the GPS receiver in order to improve the accuracy of the GPS receiver position solution, the GPS receiver has a time delay of more than 1 sec.
In the tightly-coupled structure, since the GPS receiver is not independently maintained, it is possible to design the GPS receiver so that a large time delay does not occur. In contrast, in the loosely-coupled structure, the time delay of the GPS receiver may greatly deteriorate the performance of the GPS/DR system. Further, when a vehicle moves from a GPS reception area to a non-reception area, a GPS/DR system mode is switched into a DR system mode. Here, since the accuracy of an initial velocity value is very important, the influence due to the time delay may be quite significant.
In order to reduce an error due to the time delay of the GPS receiver in the GPS/DR system having the loosely-coupled structure, the time delay must be minimized in the design of the system, or the system must be designed after the magnitude of the time delay is estimated. Accordingly, a method, which minimizes the time delay after estimating the magnitude of the time delay, is required.
The prior art usually employs a method of comparing velocity data of the DR system with velocity data provided by the GPS receiver, so as to obtain the time delay of the GPS receiver. In this way, in order to accurately measure the time delay of the GPS receiver, the velocity data of the DR system must be accurate. However, the DR system, which measures the velocity by means of the accelerometer, cannot accurately measure velocity because of the characteristics of the accelerometer. That is, since a value measured by the accelerometer contains a gravity component, it is difficult to obtain a pure acceleration component. Further, an error of the velocity obtained from the accelerometer is accumulated over time. Accordingly, because of the characteristics of the accelerometer, the DR system cannot accurately measure velocity.
Therefore, when the time delay of the GPS receiver is estimated by means of the conventional method, the measured result may be inaccurate. Further, even when the system is designed to compensate for the time delay, the performance of the entire hybrid navigation system may deteriorate.